j. p. cohoon and j. w. davidson © 1999 mcgraw-hill, inc. advanced parameter passing reference...
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J. P. Cohoon and J. W. Davidson© 1999 McGraw-Hill, Inc.
Advanced Parameter Passing
Reference parameters, const parameters, and default
parameters
Ch 7 / Foil 2
Consider
int main() {int Number1;int Number2;Number1 = PromptAndRead();Number2 = PromptAndRead();if (Number1 > Number2) {
Swap(Number1, Number2);}cout << "The numbers in sorted order:" << Number1 << ", " << Number2 << endl;return 0;
}
Ch 7 / Foil 3
Using
void Swap(int Value1, int Value2) {int Temp = Value1;Value1 = Value2;Value2 = Temp;return;
}
Ch 7 / Foil 4
Call by Reference
If the formal argument declaration is of the formptypei &pnamei then
formal argument pnamei is an alias for the ith actual argument of the function call
pnamei is a reference parameter
– Changes to the formal parameter change the actual parameter
Ch 7 / Foil 5
Reconsider
int main() {int Number1;int Number2;Number1 = PromptAndRead();Number2 = PromptAndRead();if (Number1 > Number2) {
Swap(Number1, Number2); }
cout << "The numbers in sorted order: " << Number1 << ", " << Number2 << endl;return 0;
}
Ch 7 / Foil 6
Using
void Swap(int &Value1, int &Value2) {int Temp = Value1;Value1 = Value2;Value2 = Temp;return;
}
Return statement notnecessary for void functions
Passed by reference -- in aninvocation the actual
parameter is given ratherthan a copy
Ch 7 / Foil 7
Extraction
Function to extract a value from a given stream
void GetNumber(istream &sin, int &MyNumber) {sin >> MyNumber;return;
}
Why is the stream a reference parameter?
Why is the number a reference parameter?
Ch 7 / Foil 8
Getnum.cpp
int main() {int Number1;int Number2;GetNumber(cin, Number1);GetNumber(cin, Number2);if (Number1 > Number2) {
Swap(Number1, Number2); }
cout << "The numbers in sorted order: " << Number1 << ", " << Number2 << endl;return 0;
}
Ch 7 / Foil 9
Constant Parameters
The const modifier can be applied to formal parameter declarations Const indicates that the function may not modify the parameter
void PromptAndGet(int &n, const string &s) { cout << s ; cin >> n ; s = "Got it"; // illegal assignment
} Sample invocation
int x;PromptAndGet(x, "Enter number (n): ");
Ch 7 / Foil 10
Constant Parameters
Usefulness When we want to pass an object by reference, but we do not
want to let the called function modify the object Question
Why not just pass the object by value? Answer
For large objects, making a copy of the object can be very inefficient
Ch 7 / Foil 11
Passing Constant Rectangles
void DrawBoxes(const RectangleShape &R1, const RectangleShape &R2) {
R1.Draw();R2.Draw();
}
int ApiMain() {SimpleWindow Demo("Demo Program");Demo.Open();RectangleShape Rect1(Demo, 3, 2, Blue);RectangleShape Rect2(Demo, 6, 5, Yellow);DrawBoxes(Rect1, Rect2);return 0;
}
Ch 7 / Foil 12
Default Parameters
Observations Our functions up to this point required that we explicitly pass
a value for each of the function parameters It would be convenient to define functions that accept a
varying number of parameters Default parameters
Allows programmer to define a default behavior– A value for a parameter can be implicitly passed
Reduces need for similar functions that differ only in the number of parameters accepted
Ch 7 / Foil 13
Default Parameters
If the formal argument declaration is of the form
ptypei pnamei = dvaluei then
If there is no ith argument in the function invocation, pnamei
is initialized to dvaluei The parameter pnamei is an optional value parameter
– Optional reference parameters are also permitted
Ch 7 / Foil 14
Consider
void PrintChar(char c = '=', int n = 80) {for (int i = 0; i < n; ++i)
cout << c;}
What happens in the following invocations?
PrintChar('*', 20);PrintChar('-');PrintChar();
Ch 7 / Foil 15
Default Parameters
Default parameters must appear after any mandatory parameters
Example
void Trouble(int x = 5, double z, double y) {...
}
Ch 7 / Foil 16
Default Parameters
Consider
bool GetNumber(int &n, istream &sin = cin) {return sin >> n ;
} Some possible invocations
int x, y, z;ifstream fin("Data.txt");GetNumber(x, cin);GetNumber(y);GetNumber(z, fin);
Design your functions for ease and reuse!
Ch 7 / Foil 17
Casting of Function Parameters
Calling a function is much like applying an operator to operands When a function is called
Compiler attempts to convert (coerce) the actual parameters to the types required by the formal parameters of the function
// ComputeInterest(): compute interestdouble ComputeInterest(double Principle, double InterestRate, double CompoundRate, double Years);
...
double MyInterest = ComputeInterest(100, .075, 1, 10);
Ch 7 / Foil 18
Interest.cpp
int main() {cout << "Interest is " << ComputeInterest(100, .10, 1, 10) << endl;return 0;
}
double ComputeInterest(double Principle, double InterestRate, double CompoundRate, double Years) {
double Interest = Principle * pow(1 + InterestRate, CompoundRate * Years);
return Interest;}
Ch 7 / Foil 19
Casting Parameters
Ch 7 / Foil 20
Function Overloading
A function name can be overloaded Two functions with the same name but with different
interfaces– Typically this means different formal parameter lists
Difference in number of parameters
Min(a, b, c)
Min(a, b) Difference in types of parameters
Min(10, 20)
Min(4.4, 9.2)
Ch 7 / Foil 21
Function Overloading
int Min(int a, int b) {cout << "Using int min()" << endl;if (a > b)
return b;else
return a;}double Min(double a, double b) {
cout << "Using double min()" << endl;if (a > b)
return b;else
return a;}
Ch 7 / Foil 22
Min.cpp
int main() {int a = 10;int b = 20;double x = 4.4;double y = 9.2;int c = Min(a, b);cout << "c is " << c << endl;int z = Min(x, y);cout << "z is " << z << endl;return 0;
}
Ch 7 / Foil 23
Function Overloading
Compiler uses function overload resolution to call the most appropriate function If a function definition exists where the type of the formal
parameters exactly match the types of the actual parameters, then that function definition is invoked
If there is no exact match, the compiler will attempt to cast the actual parameters so that an appropriate function definition (if any) can be invoked
The rules for function definition overloading are very complicated Advice
– Be very careful when using this feature
Ch 7 / Foil 24
Random Numbers
Generating a sequence of random numbers is often useful In a game, it ensures that a player does not see
the same behavior each time In a simulation of a complex system,
random numbers can be used tohelp generate random events
– Car crash in a simulationof a highway system
– Likelihood of a gene in cell mutation– Weather simulation
Ch 7 / Foil 25
Uniform Random Numbers
Uniform random number sequence A sequence of random numbers where
– Each value in the sequence is drawn from the same range of numbers
– In each position of the sequence, any value in the number range is equally likely to occur
Ch 7 / Foil 26
Random Numbers
Examples Generate a uniform random
number sequence in the range1 to 6
– Use a fair six-sided dice– Each roll represents a new random number
Do two die produce uniform random numbers in the range 1 ... 12?
Generate a uniform random numbersequence in the range 1 to 2
– Use a fair coin Heads: 1, Tails: 2
Ch 7 / Foil 27
Random Numbers
We can write an algorithmfor generating what lookslike random numbers
Because it’s an algorithm,we know the rules for generating the next number The generated numbers are not really random
– They are properly called pseudorandom numbers
30 21 9 28 29 ...
Ch 7 / Foil 28
Stdlib Library
Provides in part an interface to functions that generate pseudorandom numbers rand() -- returns a uniform pseudorandom number (unsigned int)
from the inclusive interval 0 to RAND_MAX Consider rand.cpp
#include <iostream>#include <string>using namespace std;#include <stdlib.h>int main() { for (int i = 1; i <= 5; ++i)
cout << rand() << endl; return 0;
}
Ch 7 / Foil 29
Different Sequences
To produce a different sequence, invoke
void srand(unsigned int); Consider seed.cpp
int main() { cout << "Enter a seed: "; unsigned int Seed; cin >> Seed; srand(Seed); for (int i = 1; i <= 5; ++i) cout << rand() << endl; return 0;
}
Ch 7 / Foil 30
Different Sequences
To get a different sequence each time Need a method of setting the seed to a random value
– The standard method is to use the computer's clock as the value of the seed
– The function invocation time() can be used Returns an integral value of type time_t Invocation time(0) returns a suitable value for
generating a random sequence
Ch 7 / Foil 31
Randseed.cpp
#include <iostream>#include <string>using namespace std;#include <stdlib.h>#include <time.h>
int main() { srand((unsigned int) time(0)); for (int i = 1; i <= 5; ++i)
cout << rand() << endl; return 0;
}
Ch 7 / Foil 32
A Factory Automation Trainer
Problem statement Design and implement a program to evaluate and train
quality control inspectors
Control Unit
Accept or reject signalVideo Image
Conveyor control signal
Video control signal
Inspector's Display Unit
Parts group onconveyor belt
Inspection stationvideo camera
Ch 7 / Foil 33
A Factory Automation Trainer
Object decomposition Video camera Display unit Control unit Parts Simulator control
Ch 7 / Foil 34
A Factory Automation Trainer
Because we do not have program-defined classes yet, wemake functions to produce object behaviors
ApiMain()
DisplayPartsImages()
GetPartsImages()
CheckResponse()
DisplaySessionResults()
Ch 7 / Foil 35
A Factory Automation Trainer
Simulation controller Get parts image Display parts images in the display window Read and record the response of the trainee Score the response Check to see if training session should end
– If time is not up, go back to step 1– If time is up, compute and print the statistics
Ch 7 / Foil 36
A Factory Automation Trainer
#include <iostream> // Program 7.10#include <string>using namespace std;#include <assert.h>#include "uniform.h"#include "rect.h"// Size of display windowconst float DisplayWidth = 14.0;const float DisplayHeight = 6.0;// Vertical position of squaresconst float YPosition = DisplayHeight / 2.0;// Length of the session (60 seconds)const long TestTime = 60 * 1000L;
Ch 7 / Foil 37
A Factory Automation Trainer
// Randomly pick one of the six colorscolor GenerateRandomColor() {
switch (Uniform(0, 5)) {case 0: return Red;case 1: return Blue;case 2: return Green;case 3: return Yellow;case 4: return Cyan;case 5: return Magenta;
}}
Ch 7 / Foil 38
A Factory Automation Trainer
// Generate the test parts by randomly// setting the color of the three partsvoid GetPartsImages(RectangleShape &P1, RectangleShape &P2, RectangleShape &P3) {
P1.SetColor(GenerateRandomColor());P2.SetColor(GenerateRandomColor());P3.SetColor(GenerateRandomColor());
return;}
Ch 7 / Foil 39
A Factory Automation Trainer
// Display the shapesvoid DisplayPartsImages(const RectangleShape &P1, const RectangleShape &P2, const RectangleShape &P3) {
P1.Draw();P2.Draw();P3.Draw();
return;}
Ch 7 / Foil 40
A Factory Automation Trainer
// Print the results from the training sessionvoid PrintSessionResults(long Time, int Attempts, int Correct, int Wrong) {
cout << "\n\nFor a training period of " << Time / 1000L << " seconds" << endl;cout << "Groups viewed: " << Attempts << endl;cout << "Correct responses: " << Correct << endl;cout << "Incorrect responses: " << Wrong << endl;cout << "Percent correct: " << Correct / (float) Attempts * 100 << endl;return;
}
Ch 7 / Foil 41
A Factory Automation Trainer// Determine if the appropriate response was givenbool CheckResponse(char Response, const RectangleShape &P1, const RectangleShape &P2, const RectangleShape &P3) {
if (P1.GetColor() == P2.GetColor() || P1.GetColor() == P3.GetColor() || P2.GetColor() == P3.GetColor()) {
return Response == 'a';}else {
return Response == 'r';}
}
Ch 7 / Foil 42
A Factory Automation Trainer
int ApiMain() {InitializeSeed();// Window for displaying the objectsSimpleWindow DisplayWindow("Training Window", DisplayWidth, DisplayHeight);DisplayWindow.Open();assert(DisplayWindow.GetStatus() == WindowOpen);
Ch 7 / Foil 43
A Factory Automation Trainer
// Print message telling user to arrange windowscout << "Please resize this window so that\n" << "that both windows are visible,\n" << "and they do not overlap.\n" << "Type any character followed by a return\n" << "when you are ready to proceed" << endl;char Response;cin >> Response;
Ch 7 / Foil 44
A Factory Automation Trainer
cout << "\n\n\n";// Create three rectangles for the three partsRectangleShape Part1(DisplayWindow, 3.0, YPosition, Blue, 2.0, 2.0);RectangleShape Part2(DisplayWindow, 7.0, YPosition, Blue, 2.0, 2.0);RectangleShape Part3(DisplayWindow, 11.0, YPosition, Blue, 2.0, 2.0);// Define objects for scoring the traineeint Attempts = 0; // Number of tests doneint CorrectResponses = 0;int IncorrectResponses = 0;
Ch 7 / Foil 45
A Factory Automation Trainer
// Record starting timeconst long StartTime = GetMilliseconds();long ElapsedTime;do {
GetPartsImages(Part1, Part2, Part3);DisplayPartsImages(Part1, Part2, Part3);
cout << "Accept or reject (a or r)? ";char Response;cin >> Response;
Ch 7 / Foil 46
A Factory Automation Trainer
++Attempts;if (CheckResponse(Response, Part1, Part2, Part3))
++CorrectResponses;else
++IncorrectResponses;ElapsedTime = GetMilliseconds();
} while ((ElapsedTime - StartTime) < TestTime);PrintSessionResults(TestTime, Attempts, CorrectResponses, IncorrectResponses);return 0;
}